1. Field of the Invention
The present invention relates to stacked integrated circuit memory.
2. State of the Art
Manufacturing methods for increasing the performance and decreasing the cost of electronic circuits, nearly without exception, are methods that increase the integration of the circuit and decrease its physical size per equivalent number of circuit devices such as transistors or capacitors. These methods have produced as of 1996 microprocessors capable of over 100 million operations per second that cost less than $1,000 and 64 Mbit DRAM circuits that access data in less than 50 ns and cost less than $50. The physical size of such circuits is less than 2 cm2. Such manufacturing methods support to a large degree the economic standard of living in the major industrialized countries and will most certainly continue to have significant consequences in the daily lives of people all over the world.
Circuit manufacturing methods take two primary forms: process integration and assembly integration. Historically the line between these two manufacturing disciplines has been clear, but recently with the rise in the use of MCMs (Multi-Chip Modules) and flip-chip die attach, this clear separation may soon disappear. (The predominate use of the term Integrated Circuit (IC) herein is in reference to an Integrated Circuit in singulated die form as sawed from a circuit substrate such as s semiconductor wafer versus, for example, an Integrated Circuit in packaged form.) The majority of ICs when in initial die form are presently individually packaged, however, there is an increasing use of MCMs. Die in an MCM are normally attached to a circuit substrate in a planar fashion with conventional IC die I/O interconnect bonding methods such as wire bonding, DCA (Direct Chip Attach) or FCA (Flip-Chip Attach).
Integrated circuit memory such as DRAM, SRAM, flash EPROM, EEPROM, Ferroelectric, GMR (Giant MagnetoResistance), etc. have the common architectural or structural characteristic of being monolithic with the control circuitry integrated on the same die with the memory array circuitry. This established (standard or conventional) architecture or circuit layout structure creates a design trade-off constraint between control circuitry and memory array circuitry for large memory circuits. Reductions in the fabrication geometries of memory cell circuitry has resulted in denser and denser memory ICs, however, these higher memory densities have resulted in more sophisticated control circuitry at the expense of increased area of the IC. Increased IC area means at least higher fabrication costs per IC (fewer ICs per wafer) and lower IC yields (fewer working ICs per wafer), and in the worst case, an IC design that cannot be manufactured due to its noncompetitive cost or unreliable operation.
As memory density increases and the individual memory cell size decreases more control circuitry is required. The control circuitry of a memory IC as a percentage of IC area in some cases such as DRAMs approaches or exceeds 40%. One portion of the control circuitry is the sense amp which senses the state, potential or charge of a memory cell in the memory array circuitry during a read operation. The sense amp circuitry is a significant portion of the control circuitry and it is a constant challenge to the IC memory designer to improve sense amp sensitivity in order to sense ever smaller memory cells while preventing the area used by the sense amp from becoming too large.
If this design constraint or trade-off between control and memory circuits did not exist, the control circuitry could be made to perform numerous additional functions, such as sensing multiple storage states per memory cell, faster memory access through larger more sensitive sense amps, caching, refresh, address translation, etc. But this trade-off is the physical and economic reality for memory ICs as they are presently made by all manufacturers.
The capacity of DRAM circuits increase by a factor of four from one generation to the next; e.g. 1 Mbit, 4 Mbit, 16 Mbit and 64 Mbit DRAMs. This four times increase in circuit memory capacity per generation has resulted in larger and larger DRAM circuit areas. Upon introduction of a new DRAM generation the circuit yields are too low and, therefore, not cost effective for high volume manufacture. It is normally several years between the date prototype samples of a new DRAM generation are shown and the date such circuits are in volume production.
Assembling die in a stacked or three dimensional (3D) manner is disclosed in U.S. Pat. No. 5,354,695 of the present inventor, incorporated herein by reference. Furthermore, assembling die in a 3D manner has been attempted with regard to memory. Texas Instruments of Dallas Tex., Irvine Sensors of Costa Mesa Calif. and Cubic Memory Corporation of Scotts Valley Calif., have all attempted to produce stacked or 3D DRAM products. In all three cases, conventional DRAM circuits in die form were stacked and the interconnect between each DRAM in the stack was formed along the outside surface of the circuit stack. These products have been available for the past several years and have proved to be too expensive for commercial applications, but have found some use in space and military applications due to their small physical size or footprint.
The DRAM circuit type is referred to and often used as an example in this specification, however, this invention is clearly not limited to the DRAM type of circuit Undoubtedly memory cell types such as EEPROMs Electrically Erasable Programmable Read Only Memories), flash EPROM, Ferroelectric, or combinations (intra or inter) of such memory cells can also be used with the present Three Dimensional Structure (3DS) methods to form 3DS memory devices.
The present invention furthers, among others, the following objectives:
1. Several-fold lower fabrication cost per megabyte of memory than circuits conventionally made solely with monolithic circuit integration methods.
2. Several-fold higher performance than conventionally made memory circuits.
3. Many-fold higher memory density per IC than conventionally made memory circuits.
4. Greater designer control of circuit area size, and therefore, cost.
5. Circuit dynamic and static self-test of memory cells by an internal controller.
6. Dynamic error recovery and reconfiguration.
7. Multi-level storage per memory cell.
8. Virtual address translation, address windowing, various address functions such as indirect addressing or content addressing, analog circuit functions and various graphics acceleration and microprocessor functions.
The present 3DS memory technology is a stacked or 3D circuit assembly technology. Features include:
1. Physical separation of the memory circuits and the control logic circuit onto different layers;
2. The use of one control logic circuit for several memory circuits;
3. Thinning of the memory circuit to less than about 50 xcexcm in thickness forming a substantially flexible substrate with planar processed bond surfaces and bonding the circuit to the circuit stack while still in wafer substrate form; and
4. The use of fine-grain high density inter layer vertical bus connections.
The 3DS memory manufacturing method enables several performance and physical size efficiencies, and is implemented with established semiconductor processing techniques. Using the DRAM circuit as an example, a 64 Mbit DRAM made with a 0.25 xcexcm process could have a die size of 84 mm2, a memory area to die size ratio of 40% and a access time of about 50 ns for 8 Mbytes of storage; a 3DS DRAM IC made with the same 0.25 xcexcm process would have a die size of 18.6 mm2, use 17 DRAM array circuit layers, a memory area to die size ratio of 94.4% and an expected access time of less than 10 ns for 64 Mbytes of storage. The 3DS DRAM IC manufacturing method represents a scalable, many-fold reduction in the cost per megabyte versus that of conventional DRAM IC manufacturing methods. In other words, the 3DS memory manufacturing method represents, at the infrastructure level, a fundamental cost savings that is independent of the process fabrication technology used.